We have continued our studies of the nature of the rim A mutation. This mutation affects the assembly of 50S ribosomes at low temperature (20 degrees), and results in the accumulation of 43S particles. A protein factor has been partially purified from wild-type cells, which can effect the partial maturation of the 43S particle in vivo. The maturation of 43S particle to "50S" particle is incomplete, since the P23S RNA is not concomitantly trimmed. Such "50S" particles are inactive in a poly U-dependent, poly-phenylalanine synthesizing reaction and are inactive in the peptidyl transferase reactions carried out in the presence of methanol. The properties of the "maturation factor" are summarized here; 1. The activity is prepared from 0.5 M NH4Cl wash of ribosomes. 2. The factor binds to DEAE at PH 7.0. 3. The factor binds to sepharose at low ionic strength. 4. Activity is heat labile (10 min at 50 C). 5. Factor is not inactivited by NEM or PCMB. Further studies of the mutant have revealed the following properties: 1. 43S particles have all 50S ribosomal proteins. 2. 43S particles are about 2% more dense than mature 50S particles in CsCl. 3. Proteins L7/L12 of the 43S particles are identical to wild-type L7/L12. 4. In vitro and in vivo synthesis of ppGpp are in the normal range. We will continue our attempts to purify the rim A maturation factor, and to study its precise role in the maturation process.